1. Field of the Invention
The present invention relates to a gas turbine engine and a method of producing the same. In particular, the present invention relates to a gas turbine engine and a method of producing the same in which a casing is prevented from abrasion by a coating provided on a surface of the casing.
2. Description of the Related Art
When a gas turbine engine is operated, rotary vanes (turbine blades) and a rotor are driven and rotated in a shroud as a casing in accordance with the expansion of a high temperature combustion gas flowing through a gas flow passage.
In order to improve the efficiency of the rotary operation by increasing the pressure of the expanding combustion gas, the clearance between the turbine blade and the shroud is as small as possible. However, both of the turbine blade and the shroud cause thermal expansion, because the combustion gas is at a high temperature as described above. As a result, the turbine blade and the shroud may slide each other.
In order to avoid the breakage of the turbine blade even when the sliding contact occurs as described above, a hard layer called “abrasive layer” is usually provided on the turbine blade, and a soft layer called “abradable layer” is provided on the shroud. The abrasive layer may be made of materials such as stabilized zirconia, and the abradable layer may be made of materials such as cubic system BN(c-BN) and Ni—Co—Cr—Al—Y alloy.
When the turbine blade and the shroud are in sliding contact with each other, the soft abradable layer is easily abraded by the hard abrasive layer. Accordingly, the force acting on the turbine blade is decreased, and hence it is possible to prevent the turbine blade from breakage. Further, because the abrasion occurs in the abradable layer, it is also possible to prevent the shroud itself from abrasion.
In relation to the technique as described above, Japanese Laid-Open Patent Publication No. 2001-303904 suggests a gas turbine in which no abrasive layer is provided for turbine blades, but abrasion margins are provided for the turbine blades so that the abrasion margins are abraded upon abutment against a coating layer of a casing. Another gas turbine is also suggested, in which abrasive layers are provided for turbine blades, while no abradable layer is provided for a casing so that the abrasion occurs on the casing.
However, when the abrasion margin disappears in the invention of Japanese Laid-Open Patent Publication No. 2001-303904, the clearance between the gas turbine and the shroud is increased. Therefore, the efficiency may be deteriorated in relation to the rotary action of the gas turbine and the rotor. Further, it is necessary to exchange the gas turbine in order to enhance the efficiency again. Therefore, maintenance cost is high.
On the other hand, when the abrasion occurs on the casing, the casing is deformed. Therefore, it is necessary to finally exchange the casing.